The present invention relates to capacitive pressure transducers. More particularly, the present invention relates to capacitive pressure transducers with improved thermal properties.
FIG. 1 shows a sectional view of a prior art heated capacitive pressure transducer 100. Transducer 100 includes several major components such as an external shell 110, a heater shell 120, a heater 130, a capacitive pressure sensor 140, a front end electronics assembly 160, a heater control electronics assembly 170, and an input/output (I/O) electronics assembly 180. As will be discussed in greater detail below, transducer 100 generates an output signal indicative of a pressure measured by sensor 140.
For convenience of illustration, many mechanical details of transducer 100, such as the construction of sensor 140 and the mounting of sensor 140 and electronics assemblies 170, 180, have been omitted from FIG. 1. However, heated capacitive pressure transducers such as transducer 100 are well known and are described for example in U.S. Pat. Nos. 5,625,152 (Pandorf); 5,911,162 (Denner); and 6,029,525 (Grudzien).
Briefly, external shell 110 includes a lower enclosure 112, an upper electronics enclosure 114, and a joiner 116 that holds enclosures 112, 114 together. Heater shell 120 is disposed within the lower enclosure 112 and includes a lower enclosure or can 122 and a cover 124. Heater 130 includes a barrel heater 132 and an end heater 134. Barrel heater 132 is wrapped around the external cylindrical sidewall of can 122 and end heater 134 is disposed on the bottom of can 122. Barrel heater 132 and end heater 134 are electrically connected via wires 136 so the two heaters 132, 134 may be simultaneously controlled via a single electrical signal. Sensor 140 and front end electronics assembly 160 are disposed within heater shell 120. Mounting posts 162 support front end electronics assembly 160 over sensor 140 and wires 164 electrically connect front end electronics assembly 160 and sensor 140. Heater control electronics assembly 170 and I/O electronics assembly 180 are disposed within the upper electronics enclosure 114. A temperature sensor (e.g., a thermistor) 190 is fixed to an internal surface of heater shell 120.
Sensor 140 includes a metallic, flexible, diaphragm 142 and a pressure tube 144. Tube 144 extends from an area proximal to the diaphragm through the heater shell 120, and through the lower sensor enclosure 112. The lower, or external, end of tube 144 is generally coupled to a source of fluid (not shown). Pressure of fluid in the source is communicated via tube 144 to the lower surface of diaphragm 142 and the diaphragm 142 flexes up or down in response to changes in pressure within tube 144. Diaphragm 142 and a reference conductive plate of sensor 140 form a capacitor, and the capacitance of that capacitor varies in accordance with movement or flexion of the diaphragm. Accordingly, that capacitance is indicative of the pressure within tube 144. Front end electronics assembly 160 and I/O electronics assembly 180 cooperatively generate an output signal representative of the capacitance of sensor 140 which is, of course, also representative of the pressure within tube 144. I/O electronics assembly 180 makes that output signal available to the environment external to transducer 100 via an electronic connector 182.
FIG. 2 shows one example of how a capacitive pressure sensor 140 can be constructed. Capacitive pressure sensors of the type shown in FIG. 2 are discussed in greater detail in U.S. Pat. No. 6,029,525 (Grudzien). The sensor 140 shown in FIG. 2 includes a circular, conductive, metallic, flexible diaphragm 142, a pressure tube 144, and an electrode 246. Electrode 246 and diaphragm 142 are mounted within a housing 248. Electrode 246 includes a ceramic block 250 and a conductive plate 252. The ceramic block 250 is rigidly mounted to the housing 248 so that a bottom face of block 250 is generally parallel to, and spaced apart from, the diaphragm. The bottom face of block 250 is normally planar and circular. The conductive plate 252 is deposited onto the bottom face of block 250 and is also generally parallel to, and spaced apart from, the diaphragm. Conductive plate 252 and diaphragm 142 form two plates of a variable capacitor 254. The capacitance of capacitor 254 is determined in part by the gap, or spacing, between the diaphragm 142 and the conductive plate 252. Since the diaphragm flexes up and down (thereby changing the spacing between diaphragm 142 and conductive plate 252) in response to pressure changes in tube 144, the capacitance of capacitor 254 is indicative of the pressure within tube 144.
FIG. 2 shows only one of the many known ways of configuring a capacitive pressure sensor 140. However, capacitive pressure sensors 140 generally include one or more conductors that are held in spaced relation to a flexible, conductive, diaphragm. The diaphragm and the conductors form plates of one or more variable capacitors and the capacitance of those capacitors varies according to a function of the pressure in tube 144.
Capacitive pressure sensors often include additional features such as a tube 260 and a getter 262 as shown in FIG. 2. When sensor 140 is being constructed, tube 260 is initially open and is used to establish a reference pressure (e.g., vacuum) in the portion of housing 248 above diaphragm 142. Once the desired reference pressure is established (e.g., by attaching a vacuum pump to tube 260), the upper portion of tube 260 is closed, or xe2x80x9cpinched offxe2x80x9d, so as to maintain the desired reference pressure inside the upper portion of housing 248. Getter 262 is often included to absorb gas molecules that get into the upper portion of housing 248 after tube 260 has been pinched off (e.g., via outgasing of electrode 250).
Returning to FIG. 1, in operation, front end electronics assembly 160 measures the capacitance (or capacitances) of sensor 140 and generates a voltage signal representative of that capacitance (e.g., the voltage signal may vary linearly with changes in capacitance). I/O electronics assembly 180 typically amplifies and further conditions that voltage signal to generate the output signal of transducer 100.
In high performance transducers, front end electronics assembly 160 must be capable of resolving very small changes in the capacitance of sensor 140 (e.g., in a sensor that measures pressures in the range of one Torr, a delta pressure of 0.1 Torr typically corresponds to a change of only two picofarads in the capacitance of sensor 140). Accordingly, it is important to minimize any stray capacitances associated with measurement of the capacitance of sensor 140. One method of minimizing these stray capacitances is to locate front end electronics assembly 160 closely to sensor 140 and thereby minimizing the length of the wires 164 that electrically connect assembly 160 and sensor 140.
Ideally, the output signal of transducer 100 varies only according to changes in the pressure of the fluid in tube 144. However, changes in the temperature of transducer 100, or temperature gradients within transducer 100, can affect the output signal. This is primarily due to the different coefficients of thermal expansion of different materials used to construct the sensor 140. A secondary effect relates to the temperature sensitive performance of front end electronics 160. Accordingly, the accuracy of transducer 100 can be adversely affected by temperature changes in the ambient environment.
To minimize the adverse effect of changing ambient temperature, the temperature sensitive components of transducer 100 (i.e., sensor 140 and front end electronics 160) are disposed within heater shell 120, and in operation the heater 130 attempts to heat the heater shell 120 to a controlled, constant temperature. Heater 130 and heater shell 120 essentially form a temperature controlled oven that attempts to maintain the temperature of the temperature sensitive components at a constant preselected value.
The construction of heaters that may be used for heater 130 is well known. One preferred heater is described in copending U.S. patent application Ser. No. 09/685,154 (Attorney Docket No. MKS-78). Briefly, heater 130 is normally formed by placing wires or traces (e.g., copper) characterized by a selected electrical resistance onto a flexible, electrically insulating, thermally conductive shell. The traces are selected so that they will heat the heater shell 120 to a preselected temperature when a particular electrical signal is applied to the traces. The electrically insulating, thermally conductive shell is commonly made from thin layers of silicone rubber or Kapton (i.e., a polyimide high temperature film sold by Dupont under the trade name Kapton). Heater 130 is normally permanently bonded to the external surface of heater shell 120 as indicated in FIG. 1.
In operation, heater control electronics assembly 170 applies an electrical signal to heater 130 via wires 172. Heater control electronics assembly 170 normally includes components for monitoring the temperature of heater shell 120 via sensor 190 and adjusting the signal applied to heater 130 so as to maintain the shell 120 at a constant temperature.
Despite the use of heater shells 120 and heaters 130 as described above, capacitive sensor 140 is still often subjected to thermal gradients. For example, in operation the top of the sensor 140 may become slightly hotter than the bottom of the sensor. Such gradients adversely affect the performance of transducer 100. U.S. Pat. No. 5,625,152 (Pandorf) discloses one structure that tends to reduce the thermal gradients on the capacitive pressure sensor. The transducers disclosed in that patent include two heater shells that are separated from one another. The capacitive sensor is disposed on one of the heater shells and the electronics are disposed in the other heater shell. Each heater shell is equipped with its own heater and the two shells may be heated to different temperatures. One problem with this structure is that providing two separate heater shells disadvantageously increases the cost of the transducer. Another problem with this structure is that it tends to increase the distance between the sensor and the electronics thereby increasing the capacitance of the wires that connect them.
Accordingly, it would be advantageous to develop structures and methods for inexpensively reducing temperature gradients in capacitive pressure transducers.
These and other advantages are provided by a thermal barrier disposed inside the heater shell between the sensor and the front end electronics assembly. The thermal barrier effectively divides the heater shell into a lower portion and an upper portion, the sensor being disposed in the lower portion. The thermal barrier and the lower portion of the heater shell form an enclosure of uniform temperature that surrounds the sensor.
Heat flowing from the electronics assembly towards the sensor is intercepted by the thermal barrier and conducted to the heater shell thereby reducing thermal gradients on the sensor. Similarly, heat flow (e.g., caused by non-symmetric placement of heaters) from the bottom of the heater shell towards the top of the shell is received by the thermal barrier thereby reducing thermal gradients in the lower portion of the heater shell.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein several embodiments are shown and described, simply by way of illustration of the best mode of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.